Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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479 USER COMMENTS

Dennis, Sakrete Sand Mix modified with Sakrete Bonder and Fortified (as a bonding agent/primer and admixture) can be used for applications up to 2” and down to ½”. Sakrete Top ‘N Bond is a polymer modified resurfacing material that can be used from ½” down to a feathered edge. You must first properly prep the area by removing any bond breakers such as dirt, dust, debris, loose material, paint, stain, oil, etc. Once the area clean and you have a solid sound substrate, brush a thin layer of Sakrete Bonder and Fortifier using an inexpensive paint brush to the area. Do not use the bonding agent with the Top N Bond.
- Chris Technical Services
Tuesday, July 28, 2015 at 11:06 AM

Where my garage door meets the floor, the slab has settled about 2” from left to right across the 12’ opening. Can I pour a 12” w x 12’l strip to level the slab? The present slab is in otherwise excellent condition. What product should I use and what prep work is needed?
- DENNIS
Monday, July 27, 2015 at 8:40 PM

Paul, for pours over 2" any one of our concrete mixes (High Strength, 5000+, Maximizer, Crack Resistant, etc.) can be used. Some of the information provided is not very clear and you may want to call in and talk to a rep for a more specific recommendation.
- Chris Technical Services
Thursday, July 16, 2015 at 4:28 PM

Romeo, once the new concrete has cured for at least 28 days you may use Sakrete Flo-Coat to resurface the area. Flo-Coat is a highly polymer modified material that aids to ensure excellent adhesion to existing concrete. You must first prep the surface properly by removing any bond breakers such as dirt, dust, loose material, etc. to ensure a quality bond. Do NOT use any bonding agents with Flo-Coat.
- Chris Technical Services
Thursday, July 16, 2015 at 3:41 PM

I had to take 6X6 posts out of a concrete patio, where the contractor poured the concrete up to the posts. I cut around them and chiseled the concrete. To replace the posts, I need to fill the holes (about 10 inches deep and 8X8 aquare to anchor the posts to the top of the patio. The new concrete will be attaching to existing 18" diameter X 24 inch deep footers. Plan was to put rebar in the old footer and extend the J Bolt to the top of the new pour. Just wondering which type of concrete and whether or not i need to use rebar. Thanks
- Paul
Wednesday, July 15, 2015 at 3:22 PM

I have just had my front entry to my house done, pad c/w steps but the brush line are crooked and there are some flaws such as trowel marks etc. This job was done by a 83 year old professional that guaranteed his work. I told him I'm not happy with the looks and he said he will apply a new top to this job and make it right. My concern is, will this new surface last and for how long. I would rather he leave's the work as is then having this repair crack and/or not bound properly in the future. Thank you for your help
- Romeo Sarrazin
Wednesday, July 15, 2015 at 11:29 AM

Lana, when pouring a new slab or walkway it is import to slope the area away from the foundation of structure to allow for proper drainage. The base of the concrete should pack down and a layer of gravel installed to provide drainage under slab. Make sure that downspouts from the gutters have the water draining away from the area by using corrugated piping.
- Chris Technical Services
Monday, July 13, 2015 at 4:49 PM

We have a new walkway being poured at our complex. Two areas fill with water every night, which has to be pumped out. Today they are blocking to get ready for the truck. What is the issue when water seeps in under the concrete? Should the walkway be sloped slightly upwards on the area where water comes in? Will the daily seepage eventually erode the stability of the walkway, causing it to tip and or crack open?
- Lana
Friday, July 10, 2015 at 10:28 AM

Tom, we recommend using Sakrete Sand Mix modified with Sakrete Bonder and Fortified (as a bonding agent/primer and admixture). Sand Mix is used for applications that require up to 2" and down to 1/2". You must first properly prep the area by removing any bond breakers such as dirt, dust, debris, loose material, paint, stain, oil, etc. Once the area clean and you have a solid sound substrate, brush a thin layer of Sakrete Bonder and Fortifier using an inexpensive paint brush. Set your pavers directly in the Sand Mix.
- Chris Technical Services
Wednesday, July 8, 2015 at 4:51 PM

I need to level a 3' by 5' concrete slab in good condition. About one half of the slab will need about 2" of concrete to level it, the other half tapering down to about 1/2 inch. I'd like to then place concrete flagstones on top. Will your Top N Bond work for this if I apply it in thin layers, building up to about 2 inches? If so, how long between applications? Or is there another Sakrete product I should use instead? thanks, Tom
- Tom McCulloch
Monday, July 6, 2015 at 7:33 PM

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